An apparatus for producing a corrected image of a region of interest is, for example, a computed tomography system, which acquires projection data and reconstructs from these acquired projection data images of a region of interest. Such a computed tomography system comprises correction units for correcting the image of the region of interest, in order to reduce artifacts in the image. In particular, such a computed tomography system can include a correction unit for reducing metal artifacts within the image. Metal artifacts arise, if the imaged region of interest contains high density regions, for example, metal implants, dental fillings or other regions of high radiation absorption. Typically, metal artifacts appear in the reconstructed image as streaks emanating from the high density region. Projection line integrals passing through the high density regions are highly attenuated leading to a substantial measurement error, and a filtered backprojection or other reconstruction methods translate this measurement error into streaky image artifacts commonly known as metal artifacts. These metal artifacts are, in particular, a severe problem in computed tomography imaging, in particular, where high density, medium density and low density image elements are present, for example, in head and neck imaging (due to dental fillings) or in orthopedic applications (due to implants).
The document WO 2005/008586 discloses a metal artifacts correction for computed tomography systems. In this correction, an intermediate image is reconstructed and the intermediate image is segmented into low density (for example soft tissue), medium density (for example bone) and high density (for example metal). The segmented high density part of the intermediate image is used to identify high density shadows in the acquired projection data, and the acquired projection data within the high density shadows are replaced by virtual projection data, which are determined by setting the image elements, i.e. e.g. the voxels or pixels, of the low and high density parts to a value depending on the low density image elements and by forward projecting this modified intermediate image yielding virtual projection data. These virtual projection data are used to fill the gaps in the high density shadows in the acquired projection data. The modified acquired projection data, i.e. the acquired projection data with the virtual projection data, which have been used to replace the acquired projection data within the high density shadows, are used for reconstructing a corrected final image of the region of interest.